A Generalized Law of Corresponding States for the Physisorption of Classical Gases with Cooperative Adsorbate-Adsorbate Interactions

The Law of Corresponding States for classical gases is well established. Recent attempts at developing an analogous Law of Corresponding States for gas physisorption, however, have had limited success, in part due to the omission of relevant adsorption considerations such as the adsorbate volume and cooperative adsorbate-adsorbate interactions. In this work, we modify a prior Law of Corresponding States for gas physisorption to account for adsorbate volume, and test it with experimental data and a generalized theoretical approach. Furthermore, we account for the recently-reported cooperative adsorbate-adsorbate interactions on the surface of zeolite-templated carbon (ZTC) with an Ising-type model, and in doing so, show that the Law of Corresponding States for gas physisorption remains valid even in the presence of atypically enhanced adsorbate-adsorbate interactions

Anomalous Isosteric Enthalpy of Adsorption of Methane on Zeolite-Templated Carbon

A thermodynamic study of the enthalpy of adsorption of methane on high surface area carbonaceous materials was carried out from 238 to 526 K. The absolute quantity of adsorbed methane as a function of equilibrium pressure was determined by fitting isotherms to a generalized Langmuir-type equation. Adsorption of methane on zeolite-templated carbon, an extremely high surface area material with a periodic arrangement of narrow micropores, shows an increase in isosteric enthalpy with methane occupancy; i.e., binding energies are greater as adsorption quantity increases. The heat of adsorption rises from 14 to 15 kJ/mol at near-ambient temperature and then falls to lower values at very high loading (above a relative site occupancy of 0.7), indicating that methane/methane interactions within the adsorption layer become significant. The effect seems to be enhanced by a narrow pore-size distribution centered at 1.2 nm, approximately the width of two monolayers of methane, and reversible methane delivery increases by up to 20% over MSC-30 at temperatures and pressures near ambient

Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage

This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3−5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ~7.3%, phosphorus ~8.1%, and nitrogen ~28.1%) had surface areas as high as 900 m^2 g^(−1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol^(−1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively

Zeolite-Templated Carbon Materials for High-Pressure Hydrogen Storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 mT2 g^(–1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g–1 (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g^(–1) (0.46 wt %) uptake per 1000 m^2 g^(–1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5–6.6 kJ mol^(–1) in the Henry’s law limit

Measurements of Hydrogen Spillover in Platinum Doped Superactivated Carbon

Hydrogen uptake was measured for platinum doped superactivated carbon at 296 K where hydrogen spillover was expected to occur. High pressure adsorption measurements using a Sieverts apparatus did not show an increase in gravimetric storage capacity over the unmodified superactivated carbon. Measurements of small samples (~0.2 g) over long equilibration times, consistent with the reported procedure, showed significant scatter and were not well above instrument background. In larger samples (~3 g), the hydrogen uptake was significantly above background but did not show enhancement due to spillover; total uptake scaled with the available surface area of the superactivated carbon. Any hydrogen spillover sorption was thus below the detection limit of standard volumetric gas adsorption measurements. Due to the additional mass of the catalyst nanoparticles and decreased surface area in the platinum doped system, the net effect of spillover sorption is detrimental for gravimetric density of hydrogen

Unusual Entropy of Adsorbed Methane on Zeolite-Templated Carbon

Methane adsorption at high pressures and across a wide range of temperatures was investigated on the surface of three porous carbon adsorbents with complementary structural properties. The measured adsorption equilibria were analyzed using a method that can accurately account for nonideal fluid properties and distinguish between absolute and excess quantities of adsorption, and that also allows the direct calculation of the thermodynamic potentials relevant to adsorption. On zeolite-templated carbon (ZTC), a material that exhibits extremely high surface area with optimal pore size and homogeneous structure, methane adsorption occurs with unusual thermodynamic properties that are greatly beneficial for deliverable gas storage: an enthalpy of adsorption that increases with site occupancy, and an unusually low entropy of the adsorbed phase. The origin of these properties is elucidated by comparison of the experimental results with a statistical mechanical model. The results indicate that temperature-dependent clustering (i.e., reduced configurations) of the adsorbed phase due to enhanced lateral interactions can account for the peculiarities of methane adsorbed on ZTC

In situ characterization of the decomposition behavior of Mg(BH4)(2) by X-ray Raman scattering spectroscopy

We present an in situ study of the thermal decomposition of Mg(BH4)(2) in a hydrogen atmosphere of up to 4 bar and up to 500 degrees C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)(2) can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 degrees C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 degrees C and 400 degrees C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 degrees C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)(2) is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.Peer reviewe